The major goal of this project is a detailed understanding of the biological and biochemical basis of Fv-1 restriction, a naturally occurring cellular system which inhibits the replication of specific N-or B-tropic murine leukemia viruses. The studies involve various cell culture techniques for quantitating virus replication and the inactivation of viruses by a variety of physical and chemical agents. Viral proteins are detected by radioimmunoassay and viral nucleic acids by transfection as well as gel electrophoresis. MuLV heated at 43 degrees C retains its ability to abrogate Fv-1 restriction, while its reverse transcriptase activity as well as its ability to replicate is almost totally inactivated. Among a series of defective MuLV mutants under study, one (clone 23) lacks both infectivity and polymerase activity but retains the ability to abrogate Fv-1 restriction. Conversely, MuLV synthesized in the presence of Actinomycin D, which lacks high molecular weight RNA, cannot abrogate Fv-1 restriction. These data confirm that abrogation represents a novel viral metabolic pathway, possibly involving the early synthesis of viral protein.